All‐Optical Writing and Erasing of Second‐Harmonic Generation in MoS ₂ for Reconfigurable Photonics

The development of reconfigurable photonic devices demands dynamic, non‐invasive control of nonlinear optical processes at the nanoscale. While second‐harmonic generation (SHG) in 2D materials (e.g., MoS 2 ) is a promising candidate, its active modulation remains challenging. Here, we report an all‐optical strategy for on‐demand writing and erasing of SHG in few‐layer MoS 2 . By alternating ultraviolet‐ozone treatment and continuous 532 nm laser irradiation, we achieve fully reversible, cyclic modulation of the SHG intensity exceeding 80‐fold. Our comprehensive spectroscopic and microscopic characterizations indicate that the tunability arises from the metastable formation and removal of surface S─O bonds, which transiently break lattice inversion symmetry. Leveraging this reversible control, we demonstrate spatially selective patterning of SHG, enabling the writing, erasing, and rewriting of optical information on a single MoS 2 flake. This work establishes a simple, non‐destructive platform for reconfigurable nonlinear photonics, with direct implications for optical memory, logic, and encryption technologies.